Somebody at EA didn't think this through, because this is spectactularly useless: multiplayer and used-sale won't mix in BC2, just like it didn't mix in BF2142. Each unique serial is irrevocably registered to your unique EA account in order to log into their Master Servers for multiplayer. No other person with another EA account can ever register that serial and log in with their EA account to play multiplayer. The unlockables at offer here are multiplayer maps only. Of course it's a one-time-use code - the whole GAME is a one-time-use code. It should still be transferable for single-player as long as either the reseller or the purchaser didn't go online for SecuROM to phone home, but the restriction on these unlockable MP maps won't mean a lick to a SP-only player. Disclosure: talking about the PC version here, obviously.
But disregard platform and think about what this means for multiplayer. With BF2 and BF2142, when they introduced new maps a year or so after release, in the Special Forces and Northern Strike expansions, they fractured/bifurcated/separated their userbase, the player community. If you had a server with all maps in rotation, players without the required expansion would simply get dropped from the server immediately. As a result the community was split and not as healthy as it could have been. What EA is doing is going for that result on Day One. Nice. Then again, I suppose in their view one "faction" of the split will be peopled totally by used-purchasers, which, y'know, screw them, right?